1. Field of Invention
This invention relates to color reproduction.
2. Description of Related Art
Generally, colors are represented in two ways, in device dependent color spaces and in device independent color spaces. To illustrate, most color display monitors, such as, for example, color computer monitors, display colors in the red/green/blue (RGB) color space, i.e., with respect to the amount of red, green, and blue that a particular displayed color contains. Using this technique, the color yellow, for example, is displayed on a color display monitor by combining a red image value of 100 percent red with a green image value of 100 percent green and a blue image value of zero percent.
Furthermore, the red, green, and blue (RGB) color values associated with the particular colors for a color display monitor are device dependent. This means that the RGB values associated with a particular color, viewed on a specific color display monitor, are unique to that specific color display monitor or, at least, to that brand of color display monitor. Simply put, because RGB color values are device dependent, if identical RGB color values, such as, for example, a red image value of 100 percent red, a green image value of 100 percent green, and a blue image value of zero percent, are input and displayed on two different color display monitors, the resulting yellow color displayed on the two color display monitors will probably not appear exactly alike.
Similarly, most color marking devices, such as, for example, color printers, print colors in device dependent terms. However, unlike most color display monitors, most color marking devices use a cyan, magenta, yellow, and black (CMYK) color space, i.e., a combination of cyan (C), magenta (M), yellow (Y) and black (K) (CMYK) to arrive at the color marking device""s printed colors. Consequently, as with RGB color values, CMYK color values are device dependent. Thus, as described above with respect to colors being displayed on color display monitors, if identical CMYK colors are printed by two different color marking devices, the printed colors will probably not appear exactly alike.
The other way of describing color is in device independent color spaces. By describing color in a device independent color space, consistent colors can be reproduced regardless of the type of device that is used to display or print the color. Therefore, color reproduction is generally done by defining colors in a device independent color space, such as, for example, L*a*b*, X Y Z, or L h v.
In an attempt to provide accurate color matching between color display devices and color marking devices, various color matching techniques have been developed that use models to translate colors from one color space to another color space. These models usually manifest themselves in the form of predetermined multi-dimensional look-up tables. These predetermined multi-dimensional look-up tables, such as, for example, a look-up table with inputs and outputs containing more than one dimension, translate colors from one color space to another color space while attempting to maintain the translated color""s perceived appearance. For example, if a user creates an image on a color display monitor and subsequently prints the created image without any color matching, the colors observed on the printed image may differ significantly from the colors originally observed on the color display monitor. However, if some type of color matching model is used, the discrepancies between the colors originally observed on the color display monitor and the colors observed on the printed image can be reduced.
Generally, to solve the problem of color matching, a printer inverse is produced for controlling colors in device dependent or device independent color spaces. Colors in the printer color space are usually in a device independent color space. The printer inverse is required to interpolate irregularly sampled multidimensional color data and is normally obtained for a particular color marking device by performing experimentation on the particular color marking device. The printer inverse is a multidimensional look-up table, preferably structured in such a way that the input nodes are regularly spaced, with the nodes located on a sequential plane.
In particular, the printer inverse is a look-up table that converts colors from device independent color spaces to device dependent color spaces, such as, for example, from L*a*b* to CMYK, or from XYZ to CMYK. For networked printers that utilize a print driver that operates in an unascertained device independent color space, the printer inverse can operate as a look-up table to convert output L*a*b* color space to input L*a*b* color space. The look-up tables are generated by measuring the printer forward transfer function between the inputs into the printer and the outputs from the printer.
For example, in a PostScript(copyright) print path, the PostScript(copyright) interpreter with colors in the XYZ/L*a*b* device independent color space becomes the input and the corresponding colors measured on a resulting color print becomes the output. The colors of the resulting color print can be measured by a color sensor, such as, for example, a spectrophotometer. The forward transfer function is then used to create the printer inverse and the associated rendering intents, such as, for example, calorimetric, pictorial/perceptual, saturation, pure, and the like.
Although the printer inverse can be generated by simply swapping the data of the forward transfer function, the data of the input grids for a printer inverse generated by merely swapping the data becomes unstructured because the output grids of the forward transfer function are unstructured. When a swapping type of printer inverse is utilized, because of the unstructured nature of the input/output (I/O) values in the swapped printer inverse look-up table, an efficient method for obtaining an effective mapping table, or printer inverse, must be used so that the relationship between the input nodes and the output nodes of the printer inverse look-up table is structured. Multidimensional interpolation is a key to obtaining such a structured mapping table.
Many techniques have been proposed for interpolating multidimensional, unstructured look-up tables. For example, Shepard""s interpolation as disclosed in Donald Shepard, xe2x80x9cA Two Dimensional Interpolation Function for Irregularly Spaced Dataxe2x80x9d, ACM National Conference Proceedings, Page 517-524, 1968. Additionally, the moving matrix method as disclosed in Raja Balasubramanian, xe2x80x9cRefinement of Printer Transformations Using Weightless Regressionxe2x80x9d, Xerox Digital Imaging Technology Center, Webster, N.Y. 14580. Furthermore, master color controls (MCC) can be used to interpolate multidimensional, unstructured look-up tables, as disclosed in U.S. patent application Ser. No. 09/083,203, incorporated herein by reference in its entirety.
The incorporated 203 application discloses a method of reducing and controlling color drift between a desired image, and an output image printed by a marking device that is intended to match the desired image, by detecting a current output color in the output image with a color sensing device. A difference between the current output color in the output image and a corresponding color in the desired image is then determined. A next output color in the output image is then automatically set equal to a corrected color that minimizes the difference between the next output color and the corresponding color in the image. This is preferably done on a real-time basis.
However, Shepard""s algorithm is time consuming and is not accurate when compared to other methods. Although computational time is not an issue in the moving matrix method, the range and the interpolation accuracy is relatively poor when compared to a master color controls (MCC) method. The master color controls (MCC) method uses an algorithm incorporating a purely control based technique with a multi-input/multi-output controller. The MCC method provides zero numerical interpolation errors when the printer I/O pairs are used in the computer as the model, and can lead to minimal errors in the printer inverse when used directly on the printer. However, the master color controls (MCC) method suffers from stability problems for input/output pairs near the printer gamut boundary.
There are many different multidimensional interpolation techniques that can be used to generate the printer inverse, such as, for example moving matrix, trilinear, tetrahedral, routines described in MATLAB(copyright) software, and sequential linear interpolation. The look-up table representing the printer inverse contains nodes that are usually located on a sequential grid and are uniformly spaced. These nodes must be generated using special techniques. The systems and methods of this invention provide an improved way of generating the nodes of the look-up table representing the printer inverse.
However, multidimensional interpolation is also required for those colors that are inside the gamut of the color image forming device but are not in the look-up table. That is, for colors that are not at the nodes of the look-up table. Thus, two processes must be used in the interpolation algorithms. The first processes generates a basic printer inverse using specialized algorithms, such as, for example, the algorithms shown below. The second processes interpolates any colors that are not in the look-up table.
Many of the current interpolation techniques do not provide an accurate printer inverse in a structured form for irregularly spaced color data. Although sequential linear interpolation (SLI) techniques can convert non-uniformly sampled data into the printer inverse, SLI techniques require an accurate, sequentially structured input grid. Thus, even sequential linear interpolation (SLI) techniques cannot use a totally unstructured input grid.
This invention provides an iteratively clustered interpolation (ICI) method useable with irregularly sampled color input data to develop a structured printer inverse. This iteratively clustered interpolation (ICI) method improves device independent color between devices, such as, for example, between printers, scanners and displays.
This invention separately provides iteratively clustered interpolation systems and methods that maintain consistent colors not only within one printer, but across a whole host of printers, monitors, and scanners.
This invention separately provides iteratively clustered interpolation systems and methods that describe a color in a device independent color space.
This invention separately provides iteratively clustered interpolation systems and methods that use L*a*b* as a color space standard.
This invention separately provides systems and methods that match an output L*a*b* color space with the input L*a*b* color space for colors inside a color image forming device gamut.
This invention separately provides iteratively clustered interpolation systems and methods that produce a printer inverse that is a uniformly sampled, structured look-up table with equally spaced grid points.
This invention separately provides iteratively clustered interpolation systems and methods that produce a printer inverse on a sequential plane in the input color space.
The accuracy of the proposed iteratively clustered interpolation (ICI) method is comparable to the accuracy of the master color controls (MCC) method for colors inside the printer color gamut. However, in some instances the iteratively clustered interpolation (ICI) method does not convert all of the data on or outside the boundary of the invertable regions of the color gamut of the color image forming device.
Therefore, in these instances, the iteratively clustered interpolation (ICI) method of this invention does not perform well when used directly in the printer. Thus, in various exemplary embodiments, the iteratively clustered interpolation (ICI) method is used in combination with the master color controls (MCC) method, to create a printer inverse with improved accuracy at the grid points inside the device independent color space.
The systems and methods of this invention provide an iteratively clustered interpolation (ICI) method that includes three steps. In a first step, an initial estimate of the printer inverse is obtained for a given set of device independent L*a*b* color values using a clustering interpolation method. The initial estimate of the printer inverse is refined using a gradient search method to improve the accuracy of the initial estimate. If the inversion of a particular color does not yield an acceptable inverse after the printer inverse is refined, the second step of the iteratively clustered interpolation (ICI) method is invoked. In a second step of the iteratively clustered interpolation (ICI) method, the initial estimate is changed appropriately and the gradient search method is applied again for this new initial estimate point. If the second step of the iteratively clustered interpolation (ICI) method fails to produce an acceptable inverse, a third step is used. In the third step of the iteratively clustered interpolation (ICI) method, the initial estimates of the printer inverse are changed by a method similar to a simulated annealing algorithm. Once the initial estimates are changed, the gradient search is used again. Thus, by using the three steps of the iteratively clustered interpolation (ICI) method outlined above, an improved printer inverse can be produced for almost any color, provided that the color is within the printer gamut. If a particular color is on or outside the printer gamut, a color inverse does not exist for that particular color.
Gamut mapping techniques not disclosed in herein can be used to map the colors outside the color gamut of the color marking device to colors inside the color gamut of the color marking device based on the specific rendering intents.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments of the systems and methods of this invention.